The present invention relates generally to optical interference filters and more particularly to a dual band optical interference filter capable of transmitting optical channels within a first and second passbands.
Optical interference filters rely on principles of interference that modify the intensities of the reflected light incident on a surface. A familiar example of interference is the colors created when light reflects from a thin layer of oil floating on water. Briefly stated, by modifying the interface of a substance and its environment with a third material, reflectivity of the substance can be significantly altered. This principle is used in the fabrication of optical interference filters. These filters can be used as one of, or as the main filtering element in optical add/drop multiplexers employed in optical communication systems to select one or more channels from a transmission signal.
In its most simple form, an optical interference filter includes a cavity which is comprised of two partial reflectors (or mirrors) separated by a spacer. The number of spacers determines the number of cavities of the filter. Each partial reflector, also referred to as a quarter-wave stack, is typically constructed by depositing alternating layers of high and low refractive index dielectric materials upon a substrate where each layer has an optical thickness (defined as: physical thickness x refractive index) of a quarter wave (xcex/4) (or odd multiple of a quarter wave) at the desired wavelength xcex0 of the filter. Exemplary high and low refractive index dielectric materials are TiO2, Ta2O5 and SiO2, respectively. The spacer is typically a half-wave (or multiple half-wave) layer of low refractive index material (e.g., SiO2). An interference filter has an associated transmission characteristic which is a function of the reflectance of the layers of high and low index materials associated with the stack.
In many applications, optical interference filters are constructed using multiple cavities. Typically, cavities are deposited on top of other cavities, with a quarter-wave layer of low index material therebetween. Multicavity filters produce transmission a spectra that are preferred in optical communication systems where steep slopes and square passbands are needed to select one or more optical channels. The larger the number of cavities employed, the steeper the slope of the transmission bandwidth associated with a particular filter. The transmission bandwidth of a multicavity filter is wider as compared with the transmission bandwidth associated with a single cavity filter.
FIG. 1 illustrates an exemplary transmission spectrum for a mirror comprising a plurality of high/low refractive index dielectric layers. The mirror exhibits high reflectivity at a stopband centered at xcex0 and rippled sidelobes including points A, B and C.
FIG. 2 is an exemplary transmission spectrum for a single cavity optical interference filter utilizing a pair of stacks each having the transmission spectrum shown in FIG. 1. As can be seen in FIG. 2 the transmission response is acceptable at wavelength xcex0 (approximately 1550 nm). However, the response at wavelength xcex1 (approximately 1310 nm) falls on the sidelobe and/or within the ripple band of the transmission spectrum, thereby making transmission of a particular wavelength in this range unreliable. More specifically, the single cavity interference filter produces high transmittance at wavelengths referenced at points A and B, but also produces relatively low transmittance as referenced at point C. Thus, transmission at a first wavelength xcex0 may be reliable while transmission for wavelength xcex1 within the ripple band or sidelobe slope are subject to variations in the transmission characteristic. This is also true for wavelengths in the 1625 nm range. FIG. 2 demonstrates that interference filters typically provide a single reliable transmission band.
As noted above, optical systems can utilize one or more interference filters to select particular channels from a transmission signal. For example, a first filter may be used to select a pay-load channel associated with voice and/or data transmission in the 1.5 xcexcm range and a second filter is used to select a service channel in the 1.3 xcexcm or 1.6 xcexcm range which carries system level and/or network monitoring information. The use of two separate filters, however, has several disadvantages. First, it increases overall system cost since it requires the manufacture and installation of two individual components. Secondly, optical networks typically have a predetermined loss budget, if exceeded, can compromise signal integrity. Each component, in this case an optical filter, contributes some loss to the overall network. By using two separate filters to select a payload channel and a service channel, each filter negatively impacts a network""s loss budget.
Thus, there is a need for a filtering element used with optical communication systems capable of selecting a first and a second optical passbands. There is a further need to provide such a filtering element which reliably selects at least one wavelength corresponding to a payload channel as well as a wavelength corresponding to a service channel within an optical network.